A. Technical Field
This invention relates generally to semiconductor manufacturing and packaging and more specifically to semiconductor manufacturing in MEMS (Microelectromechanical systems) sensing products.
B. Background of the Invention
A microelectromechanical structure (MEMS) is widely applied as a sensor to measure acceleration, rotation, pressure and many other physical parameters. The MEMS device is normally formed on a silicon substrate using a micromachining process, and thus, adopts characteristic feature sizes of several micrometers. Such miniaturized devices transduce mechanical movement to electrical signals that may indicate the level of the interested parameters. Examples of the MEMS device include accelerometers, gyroscopes, magnetometers, and pressure sensors. Various MEMS devices have been widely employed in applications ranging from common consumer products to specialized products used under extreme environments, and nowadays, they may be easily found in automotive parts, mobile phones, gaming devices, medical appliance, and military applications.
Many MEMS devices rely on capacitive sensing between a moveable electrode and a stationary electrode, and one example of such MEMS devices is a micro-machined pressure sensor. The pressure sensor measures pressure by measuring the deflection of a membrane using a capacitive read-out. The pressure sensor comprises a moveable electrode and a fixed sensing electrode, spaced by a defined gap wherein the movable electrode deforms in response to the pressure difference between the external pressure and a reference pressure in a sealed cavity. The capacitive change may be induced by variation of the capacitive gap or area of the sensing capacitor that is associated with the relative location change between the electrodes.
Thermo-mechanical stress may produce deformation of both electrodes in different way, and ultimately, lead to an offset or sensitivity drift to the sensing capacitor (sensor interface circuit) even though no pressure difference is applied to induce any capacitive change. In an ideal situation, the capacitive variation of the sensing capacitor should only be associated with the pressure difference, and does not exist when no pressure difference occurs.
However, thermo-mechanical stress may be accumulated in the MEMS device during the course of manufacturing, soldering, packaging and device aging. Non-uniform stress can build up within the substrate and the device structure including the membrane, and unavoidably cause the substrate to warp and the membrane to shift or to be deformed. The sensing output from a sensor interface circuit may reflect such displacements resulting from the non-uniform thermal stress, and lead to an offset value and a sensitivity drift for the sensed pressure.
Device performance of a capacitive pressure sensor is compromised due to the thermo-mechanical stress. Such performance degradation is commonly shared by the MEMS devices that primarily rely on membranes and capacitive electrodes for transducing and sensing mechanical movement. There is a need to compensate or reduce the impact of the thermo-mechanical stress that builds up during the course of manufacturing, packaging, assembly and regular operation.
Pressure sensor performance can vary based on stress sensitivity and temperature. A pressure sensor measures pressure by measuring the deflection of a MEMS membrane using a capacitive read-out method. Temperature and package stress can produce deformation of both electrodes in different ways, causing capacitance variation even without external pressure variation. These deformations impact sensor accuracy. A prior art solution is to reduce the impact of deformation by introducing a trench isolation inside the package surrounding sensing element. However, this method still produces inaccuracies and increases complexity and cost of the package itself.
FIG. 1 shows two cross sectional views of a MEMS pressure sensor illustrating deformation in the prior art solutions 100 and 170. FIG. 1 example 100 shows pressure sensing membrane 105, substrate 115, package 110, sealed cavity 120, and fixed sensing electrode 125. In example 100, there is no package stress or substrate. FIG. 1 example 170 shows substrate deformation and includes pressure sensing membrane 130, deformed substrate 140, package 135, sealed cavity 145, and sensing electrode 150. As is understood by one of ordinary skill in the art, FIG. 1 example 170 illustrates that the substrate deformation 140 also leads to sensing electrode deformation 150.
FIG. 1 example 170 shows the sensing membrane 130 and the cavity underneath it being hermitically closed 145. A fixed sensing electrode 150 is used to detect capacitance variation due to membrane deformation under applied pressure. On this kind of device when a substrate deformation due to package stress or temperature occurs, capacitance variation due to undesired fixed electrode or membrane deformation can be detected even without applied pressure, causing loss in sensor accuracy.
In summary what is needed is a solution for a MEMS pressure sensor reduces the unwanted effect of thermo-mechanical stress.